This type of vacuum booster for a vehicular brake is known from JP-B-3759451 etc. In this type of vacuum booster, the front shell half is formed with, in an integral manner, a cylindrical circumferential wall whose rear end portion is connected to the rear shell half, an end wall which is continuous with the front end of the circumferential wall and whose diameter decreases as the position goes forward, a flat attachment portion which is continuous with the front end of the end wall, and a fitting cylindrical portion which extends rearward from a central portion of the attachment portion so as to be able to be fitted with the rear end portion of the cylinder body of the master cylinder. A connection portion of the attachment portion and the end wall is stepped in an imaginary cross section including the axial lines of the pair of bolts for screwing the front shell half to the flange of the cylinder body, and the end wall is straight in a cross section that is perpendicular to the above imaginary cross section. The strength of the front shell half is increased in this manner.
However, in the shell structure disclosed in JP-B-3759451, the flat attachment portion basically has a rhombic shape whose four corners are rounded. When local tensile loads act on the bolts by which the attachment portion is screwed to the flange of the cylinder body, the attachment portion may be deformed plastically because the areas of the flat portions around the bolt screwing portions are relatively large. To prevent this phenomenon, it is necessary to set the attachment portion relatively thick, resulting in weight increase.